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The troy ounce (ozt) is 480 grains, somewhat heavier than an avoirdupois ounce (437.5 grains).[2] A grain is exactly 64.79891 mg; hence one troy ounce is exactly 31.1034768 g, about 10 percent more than the avoirdupois ounce, which is exactly 28.349523125 g. The troy ounce is the only ounce used in the pricing of precious metals, gold, platinum, and silver. The grain, which is identical in both the troy and avoirdupois systems, is used to measure arrow and arrowhead weights in archery and bullets and powder weights in ballistics. Grains were long used in medicine but have been largely replaced by milligrams.

There are 12 troy ounces per troy pound,[2] rather than 16 avoirdupois ounces (oz) in the avoirdupois pound (lb) as in the more common avoirdupois system. The avoirdupois pound is 147⁄12 (≈ 14.583) troy ounces.

In Scotland the Incorporation of Goldsmiths of the City of Edinburgh used a system in multiples of sixteen. (See Assay-Master's Accounts, 1681-1702, on loan from the Incorporation to the National Archives of Scotland.) Thus there were 16 drops to the troy ounce, 16 ounces to the troy pound, and 16 pounds to the troy stone. The Scots had several other ways of measuring precious metals and gems, but this was the common usage for silver and gold.

The troy system was the basis for the pre-decimalisation British system of coinage introduced by Henry II of England, in which the penny was literally one pennyweight of silver. One pound sterling was equal to twenty shillings, with each shilling equal to twelve pennies. Thus, one pound sterling equals 240 pennyweights, or one troy pound of sterling silver.